The invention relates to a composite structure for microlithography, in particular a holding device for a wafer, comprising two or more components, the surfaces of which are bonded together at least at one bond. The invention further relates to a wafer stage and to an optical arrangement, in particular a projection illumination apparatus for microlithography, comprising such a composite structure or such a wafer stage, respectively.
In microlithography, devices (e.g. wafer chuck, wafer stage, wafer table) are required, for example, for holding a wafer, and, although these can be implemented in part as monolithic, they are relatively heavy as a result of the overall sizes required there. Hence, a light-weight structure is frequently desired in such applications, i.e. a structure having cavities for reducing the weight. Such a structure can be implemented as a composite structure which is composed of a plurality of, e.g. ceramic, mineral, glass or glass-ceramic components. In this case, high requirements are imposed on the composite structure and in particular, on the joint, i.e. the bond between the surfaces of the components which are bonded together. The latter should be watertight and long-term stable, i.e. it should exhibit no change in the coefficient of thermal expansion under the action of temperature and in addition, no drift; the same applies to the materials of the components to be bonded together. A composite structure for microlithography is understood in the sense of this application as a composite structure in which the bond satisfies high requirements at the joint even at high beam intensities such as are usual in microlithography. It is understood that the composite structure is also suitable for other systems in which high beam intensities occur, e.g. for laser processing systems which are used, for example, in so-called “annealing” methods.
Various methods for producing composite structures of the type specified initially have been disclosed in the literature. For example, US 2004/0247826 A1 discloses the production of composite structures which are used, for example, in microlithography by joining together at least one glass ceramic component with another component. The composite structures, also designated as light-weight structures, have a fixed bond (“joint”) between the surfaces which are joined together, which should withstand a tensile stress of more than 4000 psi and is also temperature-stable. This bond is produced by curing a silicate-containing joining agent at room temperature or under heat treatment with a predefined slow increase in temperature to dehydrate the bond.
The method described there for the bonding of glass ceramics at low temperatures has the advantage over the bonding of glass ceramic parts by fusing on reaching or exceeding the transition temperature of the glass phase (so-called “fusion bonding”) that in this case, no deformations can occur as a result of the increase in the viscosity of the glass near the transition temperature. Compared to conventional methods for bonding components at low temperatures, e.g. by means of epoxy resin, the method described there also has the advantage that no organic joining agent is used. When such organic joining agents are used, pyrolysis and/or photolysis of the bond may occur in high-power applications, inter alia, on exposure to laser light in the UV or EUV wavelength range, e.g. at 193 nm, so that this becomes unstable. Another problem is possibly also the matching of the refractive indices of the components as a result of the relatively thick boundary layer between the surfaces.
Known from U.S. Pat. No. 6,284,085 B1 is a method in which two materials are bonded by hydroxide-catalysed hydration/dehydration at room temperature by applying hydroxide ions to at least one of the two surfaces to be joined before the surfaces are brought sufficiently close together that a chemical (covalent) bond is formed between them. For this purpose, the surfaces can be brought sufficiently close together by placing one component on the other. Furthermore, a silicate-containing material can be used as filling material to fill intermediate spaces between the surfaces caused by surface unevennesses, whereby the silicate-containing material can also be in powder form. The hydroxide-catalysed bond should be as strong and reliable as a high-temperature bond and as precise and transparent as an optical contact bond.
In addition to amorphous materials such as quartz glass (“fused silica”) and glass ceramics (e.g. Zerodur), crystalline materials, in particular laser crystals such as for example, yttrium aluminum garnet (YAG), may also be mentioned as materials which can be bonded by means of the method described above.